In digital processing systems there is a need for reliable means of transmitting data between processing entities. The transmission of data having a relatively slow repetition rate, e.g., 5 Khz, presents only minor problems, however, as the repetition rate of the data increases the problems of transmission and reception become greater. Additionally, where it is necessary to recover timing information from the data as it is received, it is essential that the serial data contain distinguishable synchronizing information. This implies that the bandwidth of the received message, comprising both data and synchronizing information, is greater than the bandwidth necessary to convey the information content of the data alone. Furthermore, in many applications of data transmission systems, it is essential that the integrity of the information content of the data message be maintained. It is common practice to insert error checking bits, e.g., parity information, in the serial data message so that the receiving apparatus can detect apparent errors in the received message content. The insertion of parity bits in a serial data stream further increases the bandwidth requirement of the transmission facilities.